Add an atomic rename() function that allows replacing existing files

Add an atomic rename() function, so that existing files can be replaced with new ones. The Unix way is:
::rename(old, new); // "new" is an existing file

or:
::link(old, new);
::remove(old);

on Windows, it requires using MoveFileEx with MOVEFILE_REPLACE_EXISTING. (We're currently using MoveFile)

See http://msdn.microsoft.com/en-us/library/aa363875(VS.85).aspx for the example that ideally is possible for Qt to support.